1. Field of the Invention
The present invention relates to a safety valve and a nonaqueous electrolyte secondary battery using the same.
2. Description of the Related Art
In recent years, portable information devices such as lap-top computers and wordprocessors, AV devices such as camera integrated video tape recorders and liquid crystal television sets, and mobile communication devices such as portable telephones are remarkably developed. For batteries used as power supplies, secondary batteries having small sizes, light weights, and high energy densities are demanded. Until now, aqueous-solution-based secondary batteries such as a lead battery, a nickel-cadmium battery, and a nickel-hydrogen battery are used. These aqueous-solution-based secondary batteries sufficiently satisfy the demands related to light weights and high energy densities.
Recently, as clean batteries having high energy densities, nonaqueous electrolyte secondary batteries attract considerable attentions and are greatly expected.
A conventional nonaqueous electrolyte secondary battery will be described below with reference to FIGS. 4 to 6.
FIG. 4 is a sectional view showing a conventional nonaqueous electrolyte secondary battery (e.g., disclosed in Japanese laid-open patent publication No. 8-315798).
In the nonaqueous electrolyte secondary battery, an electrode element 2 is a cylindrical bottomed outer packaging can 1 holds an electrode element 2 therein, a nonaqueous electrolytic solution (not shown) is injected into the outer packaging can 1, and the nonaqueous electrolytic solution soaks in the electrode element 2.
The electrode element 2 is constituted such that a positive electrode and a negative electrode each formed by a mixture obtained by mixing an active material, a binder, and a conductor with a elongated current collector are-laminated across a micro-porous separator as positive electrode and negative electrode, and the laminated structure is winded around, e.g., a cylindrical core in the form of a spirally coiled electrode.
The electrode element 2 is inserted into the outer packaging can 1 such that the leading side of a negative electrode lead 10 faces the bottom side of the outer packaging can 1.
On the both sides of the electrode element 2, insulation plates are arranged, and free ends of the leads 9 and 10 of the electrode element 2 are led to the outside of the insulating plates. The free end of the negative electrode lead 10 is welded on the bottom surface of the outer packaging can 1 serving as an electrode terminal leading portion.
A lid 7, a PTC element 3, and a safety valve 6 are caulked on one end side of the outer packaging can 1 through a gasket 8 to seal one end of the outer packaging can 1.
At the central portion of the safety valve 6, a projecting portion 6a projecting toward the electrode element 2 is formed. The projecting portion 6a is welded on a sub-disk 4 welded on the free end of the positive electrode lead 9. In this manner, the projecting portion 6a is electrically connected to the positive electrode lead 9 of the electrode element 2.
A safety valve used in a conventional nonaqueous electrolyte secondary battery will be described below with reference to FIGS. 5 and 6.
The configuration of the safety valve will be described below. FIG. 5A is a sectional view showing an action of the safety valve in a normal state of the conventional nonaqueous electrolyte secondary battery. FIG. 5A shows the upper portion of FIG. 4.
FIG. 6 is a plan view and a sectional view which show the configuration of the safety valve used in the conventional nonaqueous electrolyte secondary battery in a normal state. As shown in FIG. 6, a linear thin portion 6c is formed almost along a circle centering on the projecting portion 6a. In addition, four thin portions 6d extending in the radial direction are formed outside the linear thin portion 6c. 
The disk 11 is fixed on the inner side of the safety valve 6 through a disk holder 12.
The shape of the disk 11 will be described here. FIG. 4B is a plan view of a disk used in a conventional nonaqueous electrolyte secondary battery.
In FIG. 4B, an edge portion 11a is a belt-like plate which partially constitutes the disk 11 and has a circular shape at the outside of the plate. The outer edge portion 11a itself is fixed to the gasket 8 to support the disk 11 as a whole.
A depressed portion 11b partially constitutes the disk 11. The shape of the depressed portion 11b is a flat plate and is connected to the edge portion 11a. 
The disk 11 has a central hole 11c. The central hole 11c is a circular hole centering the symmetrical point of the disk 11.
A peripheral hole lid is an almost rectangular hole having a semicircular portion on the central hole 11c side, and the central axis of the peripheral hole 11d is in the radial direction.
The sub-disk 4 shown in FIG. 4A has a thin-disk-like shape, and is welded on the electrode element 2 of the disk 11 at the center of the disk 11.
A positive electrode lead 9 is welded on the electrode element 2 of the sub-disk 4. In this manner, the sub-disk 4 and the positive electrode lead 9 are electrically connected to each other.
The operation of the safety valve will be described below with reference to FIGS. 5 and 6. In this case, the safety valve 6 has two mechanisms, i.e., a current cut-off mechanism and a cleavage mechanism.
An operation in the current cut-off mechanism will be described below. FIG. 5B is a sectional view showing an action of a safety valve in a current cut-off state in the conventional nonaqueous electrolyte secondary battery.
When a gas is generated in the outer packaging can 1 for some reason, the internal pressure increases. At this time, the generated gas passes through a hole existing near the outer periphery of a disk 11 to pressurize the internal surface of the safety valve 6. In this manner, the safety valve 6 is transformed outside.
In FIG. 2A, although the sub-disk 4 closes the central hole 11c of the disk 11, the diameter of the sub-disk 4 is small. For this reason, the hole formed near the outer periphery of the disk 11 is not closed by the sub-disk 4. Since the peripheral hole 11d of the disk 11 is not closed as described above, a gas existing in the battery can pass through the disk 11. In contrast to this, since the safety valve 6 has no hole, the gas existing in the battery cannot be discharged outside, and an airtight state is kept.
In addition, at the welded portion between the projecting portion 6a of the safety valve 6 and the sub-disk 4, the sub-disk 4 existing around the welded portion is torn by shearing force. In this manner, when the projecting portion 6a and the sub-disk 4 are separated from each other, an electric connection between the positive electrode lead 9 of the electrode element 2 and the lid 7 is cut.
Here, the transformation of the safety valve 6 will be further described. As shown in FIG. 5B, when the safety valve 6 is transformed, the safety valve 6 is largely transformed at positions 6k and 6l. More specifically, the position 6k indicates the outer periphery of a flat region inside the safety valve 6, and the position 6l indicates a position which is very close to the projecting portion 6a. The position 6l which is the bending point of these portions corresponds to the portion of the thin portion 6c in FIG. 6A. Since the portion of the thin portion 6c is mechanically weakest, the thin portion 6c is maximally transformed by pressure.
In addition, due to the transformation of the safety valve 6, at the welded portion between the projecting portion 6a of the safety valve 6 and the sub-disk 4, the sub-disk 4 existing around the welded portion is torn by shearing force. In this manner, when the projecting portion 6a and the sub-disk 4 are separated from each other, an electric connection between the positive electrode lead 9 of the electrode element 2 and the lid 7 is cut.
As is apparent from FIG. 5B, the distance between the bending points 6k and 6l is large. For this reason, due to the transformation of the safety valve 6, the projecting portion 6a is largely separated from the sub-disk 4. In this manner, since the projecting portion 6a and the sub-disk 4 are largely separated from each other, a current cut-off operation can be reliably performed.
An operation in the cleavage mechanism will be described below. FIG. 5C is a sectional view showing the action of a safety valve of the conventional nonaqueous electrolyte secondary battery in a cleavage state.
When the pressure in the outer packaging can 1 is higher than the pressure in the current cut-off state, the safety valve 6 itself is cleaved so that a generated gas is released through a ventilation hole formed in the lid 7.
A cleaving operation of the safety valve 6 will be described below with reference to FIG. 6B. FIG. 6B includes a plan view and a sectional view showing a cleaving manner of a safety valve used in a conventional nonaqueous electrolyte secondary battery in a cleavage state.
Here, a series of cleaving-operations is performed in the following order. More specifically, first, the thin portion 6c along the circle is cleaved along the groove of the thin portion 6. Tensile force in a direction perpendicular to the direction of the groove acts on the thin portions 6d which are radially formed, and the thin portions 6d are cleaved along the groove. In this case, as shown in FIG. 5C, the thin portion 6c which is not cleaved may partially remain.
As described above, when the pressure in the outer packaging can 1 increases, the thin portion 6c and the thin portions 6d are continuously cleaved. For this reason, the generated gas can be discharged outside after the safety valve 6 is cleaved.
However, in the conventional nonaqueous electrolyte secondary battery, although the thin portion 6c is almost completely cleaved, the thin portions 6d are only half cleaved from the center of the safety valve 6, and the widths of the cleavages are also small. More specifically, the passage area for the generated gas is only an area corresponding to a separate portion 6e and the small gaps of the thin portions 6d formed the circumference. Therefore, when a gas is generated in the outer packaging can 1, the conventionally used safety valve cannot release the generated gas outside within a short time.
As a solving means for the problems, an increase of the opening area of the peripheral hole 11d can be considered. However, the opening area is increased, the mechanical strength of the disk 11 itself cannot be easily assured. In order to assure the mechanical strength, the thickness of the disk 11 may be increased. However, when the thickness is increased, the capacity of the battery must be decrease. For this reason, it is actually difficult to increase the opening area of the peripheral hole 11d. 